In directly addressed electrochromic displays each pixel is connected by a separate electric conduction line to an external drive voltage source, facilitating simultaneous individual electrical control of all pixels in the display. When the number of pixels in a display is large, it is either physically impossible or impractical to connect one separate line to each pixel. To overcome this problem the pixels are commonly arranged in a matrix structure in which they are addressed by time-multiplexing techniques via row and column lines from the matrix edges. Such displays and the methods of addressing them are denoted matrix displays and matrix addressing, respectively.
In active-matrix displays each pixel is equipped with a respective electronic circuit with at least one transistor, in which the conduction state of each transistor controls the colour state of the respective pixel.
Matrix displays without a separate transistor for each pixel are called passive-matrix displays. The pixel circuitry in active-matrix displays improves the addressing properties of the pixels, however, the manufacturing of active-matrix displays is more complicated and expensive, compared to passive-matrix displays. On the other hand, several drawbacks such as slow response time, image diffusion and cross-talk have limited practical applications of passive-matrix addressed electrochromic devices. The image diffusion can be classified as a cross-talk effect in a broad sense. It is a phenomenon which is an interaction between pixels through the electrolyte, and causes a slight coloration of neighboring pixels on the chosen row and column. The undesirable coloration decreases with increasing distance from the selected pixel. A commonly used technique for minimizing the image diffusion effect is a physical isolation of the individual display pixels from each other.
Another source of cross-talk is an interaction between pixels via the electrodes associated with the matrix. I.e. not only the pixel at the intersection of the addressed pair of a column and a row changes its color, but also other neighboring pixels. Cross-talk occurs because a voltage difference is present also across the electrolyte in pixels surrounding to the addressed pixel. Moreover, an electrically isolated colored display pixel normally has a different voltage across its electrodes compared to a clear non-charged pixel. When two pixels are charged into different colour states and connected together, the colored pixel tends to drive or colour the clear pixel through capacitive discharging.
One example of an electrochromic passive matrix display is described in WO 2004/114008. The pixel comprises an electrolyte sandwiched between two organic layers, each comprising electrochromic polymers. Further, the pixel is sandwiched between two electrodes. Moreover, the passive matrix display is formed by adding a layer with diode character between one of the electrodes and the organic layer. In other words, a threshold voltage has to be exceeded before the pixel may alter its color. Hence, the amount of cross-talk in the display is reduced. Besides introducing a layer with diode function in the pixel, the application also suggests that a passively addressable pixel may be achieved by a careful combination of pixel materials, but absolutely no information or examples are given as to how these materials should be selected in order to achieve this effect.